The relatively low hardness of platinum and gold alloys is a major limitation in their use in jewellery and watch making, essentially due to their proneness to wear and scratching which degrades the visual appeal of items made thereof.
A second difficulty associated with objects made of platinum by casting is the inherently high melting point of the currently used platinum alloys. This entails low volume casting trees and special refractory materials for mould making. Significantly reducing the melting temperature of platinum alloys for use in jewellery and watch making would be therefore of interest.
Typical gold and platinum alloys have a hardness below 300 HV and 200 HV, respectively. Some less standard grades of hardenable Pt-alloys mainly with Zr, Ti and Ga as alloying elements reach hardness up to 421 HV [1].
Described in the literature are the binary eutectic alloys of Pt—Si and Pt—B with typically 2-5 wt % of alloying additions having a hardness of 440 HV and 327 HV, respectively [2].
Known to the state of the art are further bulk metallic glasses based on Pt with a hardness “around 400 HV” [3, 4]. These alloys are essentially quartenary or higher order alloys derived from the Pt—P system with additional alloying elements to maintain the glassy state in the alloy at low cooling rates and concomitantly to larger cross sections [5]. Due to these alloying elements the overall Pt content is typically close to 850/1000 and thus below the level of generally accepted jewellery grade Pt which is 950/1000 in Europe and 900/1000 in the US. In an effort to comply with the 950/1000 standard (see Ref [3]), an alloy has recently been described in the literature where a small fraction of the main alloying element phosphorus is replaced by 4 and 1.5 at.-pct of B and Si, respectively, yielding a hardness of 395 HV [6].
Japanese patent application JP 1985/0268628 [7] furthermore discloses a high hardness Pt alloy containing 1.5-6.5 wt.-pct Si and several wt.-pct of alloying elements of the group Pd, Cu, Ir, Rh, Au, Ag, Ni, and Co. The hardness is up to 580, 620 and 630 HV for alloys complying with the Pt 950/1000, 900/1000 and 850/1000 standard, respectively. Analysing the data from this prior art shows that:                i) The hardness is first depending on the silicon content increasing strongly up to about 4 wt.-pct Si, corresponding to the binary eutectic        ii) For a given Si content increasing the content of a ternary alloying element, e.g. Cu from 7 to 12 wt.-pct, has only little effect on the hardness.        iii) The addition of as little as 1 wt.-pct of Cu to the eutectic composition changes the hardness from 440 HV [2] to 580 HV.        
Known are furthermore surface treatments of Pt and their alloys by creation of a diffusion layer in which the alloys are hardened by letting Ga and B diffuse into the Pt base metal [8, 9]. Surface hardness values of up to 385 HV and 700 HV for Ga and B, respectively have been disclosed [8]. In the case of the B diffusion layer the hardness is explicitly mentioned to be derived from including the B as interstitial solid solution in the Pt crystals. However, the cited concentrations of B in Pt are difficult to conciliate with Pt—B solid solution as claimed to be the reason for the high hardness in that patent [9].